The injector is a solenoid injector, of the urea injector type, used in an exhaust line of a motor vehicle and situated upstream of a nitrogen oxide catalyst, of SCR (Selective Catalyst Reduction) type.
The injector injects urea, more specifically an aqueous solution containing 32.5% by volume of urea, in the exhaust line. The duly injected urea is propagated to the SCR catalyst, to react with the nitrogen oxides (NOx) originating from the combustion chamber of the engine and present in the SCR catalyst. The chemical reaction produces dinitrogen and oxygen at the exhaust line outlet.
Such an SCR system is known to those skilled in the art and makes it possible to considerably reduce the amount of nitrogen oxide emitted by the engine with which the vehicle is equipped. A major drawback in implementing an SCR system lies in the correct dosage of the quantity of urea to be injected into the SCR catalyst. In practice, an excessively small quantity of urea injected does not make it possible to reduce the nitrogen oxides present in the exhaust in satisfactory proportion. The international standards regarding the maximum amount of nitrogen oxide (NOx) emissions at the exhaust outlet are then no longer observed. An excess quantity of urea injected results in odorous and irritant leaks of ammonia at the exhaust pipe outlet, which is not desirable.
Controlling the quantity of urea that is injected by the injector is therefore of prime importance. To this end, it is necessary to rapidly detect its malfunction, more particularly to detect a blocked injector. Such injector failures can occur, for example, when the aqueous solution of urea crystallizes inside the injector at high temperature (>100° C.) In fact, above 100° C., the water in the aqueous solution is evaporated, and the urea forms solid residues. It is possible to melt these residues by heating the urea above its melting point, that is to say, above 140° C.
In order to detect the malfunctioning of a urea injector, it is known practice from the prior art to position a nitrogen oxides (NOx) sensor downstream of the SCR catalyst. This sensor measures the level of nitrogen oxides present at the outlet of the SCR catalyst and makes it possible to detect a failing urea injector. However, in the case of an injector that is blocked open, a significant quantity of ammonia is first stored in the SCR catalyst until the latter saturates, and the NOx sensor does not instantaneously detect an abnormally high quantity of ammonia due to the leak from the injector. Nor, in the case of an injector that is blocked closed, does the NOx sensor fairly rapidly detect an abnormally high quantity of NOx due to the failure of the injector, because of the inertia of the chemical reaction occurring inside the SCR catalyst.
Furthermore, the detection of the malfunctioning of the urea injector by virtue of the NOx sensor does not make it possible to diagnose a failure of the injector before the SCR system is started up. In practice, it is necessary first to activate the injector several times by urea injection commands and wait for the response from the NOx sensor, situated downstream of the SCR catalyst, before being able to analyze said signal. With such a method, leaks of ammonia and excessive quantities of NOx in the exhaust during a short instant before the diagnosis is made are inevitable.
It is also known from the prior art, in order to detect a blocked solenoid injector, for example a fuel injector, to measure a voltage at the terminals of the injector. US 2012/0296553 A1 describes a control system for an internal combustion engine and a method for controlling said engine that makes it possible to distinguish a blocked fuel injector from a “normal” injector, that is to say one that is operating correctly. The distinction is made on the basis of the measurement of a voltage at the terminals of the injector. During the command to close or open the injector, if the latter is blocked, the voltage at the terminals of the injector deviates from a nominal voltage.
On the same principle, it is known practice to measure a voltage Ur (see FIG. 3a) at the terminals of a measurement resistance in order to determine whether or not the injector is blocked. FIG. 3a shows an injector 10 and a control device D for said injector 10 from the prior art. The injector is, on one side, supplied with current (generally a voltage E of 12 V) by a microcontroller 80 and, on the other side, connected to the ground. A measurement resistance r is connected on one side to the injector 10 and on the other side to the ground. The voltage Ur at the terminals of the measurement resistance r is measured by the microcontroller 80. Upon a command to open the injector 10, the voltage Ur increases asymptotically (see curve B in FIG. 1), but undergoes a temporary decrease Z when the injector 10 finally opens. When the injector 10 is blocked (curve A), this temporary decrease Z does not appear on the measurement of the voltage Ur. The presence or the absence of this temporary decrease Z consequently makes it possible to rapidly determine whether or not the injector is blocked.
These methods from the prior art present drawbacks, they do not make it possible to diagnose a failing injector before starting up the system with which it is associated (in our example, the SCR system).
It will be understood that it is necessary to rapidly detect the malfunctioning of a urea injector and do so even before the SCR system is started up. Such is the aim of the present invention.